I've been making games professionally across a wide gamut of technologies for over sixteen years and that's simply not true. Further there are loads of games written in JS that prove it's not true! Believe it or not I've actually written game code in languages SLOWER than JS for shipping titles!
Krunker (https://krunker.io/) is a browser based multiplayer shooter.
CrossCode (http://www.cross-code.com/en/home) is another pretty famous one that is also on Steam and very popular.
Slither.io (http://slither.io/) and really the whole genre that these sorts of games became.
The whole Godot Engine IDE (https://editor.godotengine.org/releases/latest/) has a web version.
Game Dev Tycoon (https://store.steampowered.com/app/239820/Game_Dev_Tycoon/) is made with JS.
Curious Expedition (https://store.steampowered.com/app/358130/Curious_Expedition...) was written in CoffeeScript which is compiled to JS.
And so on, and on. There are literally loads including umpteen multiplayer ports of Quake 3 and that sort of thing. Throw in all the JS13k games. Numerous games people have published using the Unity web exporter and so on. All the little web games people have released on itch.io, all the stuff made with Twine and Bitsy. And on, and on.
The idea that JS is too slow to make games with is pretty silly and smacks a bit of willful ignorance.
These prove the point, they don't scale or deliver anything that will dislocate roblox. Even wormate (which is way better than slither does not run properly on my android pad)...
> Javascript is not usable, you wont be able to make anything useful if that is your underlying VM.
And yet all these useful games exist all targeting JS backends. Some of which have made oodles of cash.
Of that list I've been making games for dot big bang for the last three years.
For other slower languages I worked in for shipping games there is EVE Online a large part of which is written in Python and then APB which had a lot of code written in UnrealScript for Unreal Engine 3 both of which are very much definitely slower than JS.
Edit: Another really cool browser based project is this fight simulator: https://www.geo-fs.com/